Near-infrared (NIR) imaging using endoscopes has been described in the literature for various clinical applications. Typically, such an imaging modality utilizes a contrast agent (e.g. indocyanine green) that absorbs and/or fluoresces in the 700-900 nm range of the NIR. In some endoscopic imaging systems capable of high resolution simultaneous color and NIR imaging, none of the image sensors (if multiple image sensors are used) or specific pixels of an image sensor (if only a single color image sensor is used) are exclusively dedicated to NIR imaging. One exemplary imaging system, described in the Annex of the present disclosure, utilizes a red, green, blue (RGB) sensor assembly to acquire both color and NIR fluorescence images by employing the red image sensor to, alternately and in rapid succession, acquire both the red light required for the color image and NIR light required for the NIR image. This imaging system is intended to be used in conjunction with image-projecting optical instruments such as endoscopes, microscopes, colposcopes, etc. that have also been optimized for both visible light and NIR imaging applications. Specifically the optical instruments (i.e. endoscopes, microscopes, colposcopes, etc.) and the optical assemblies (optical couplers) that couple these instruments to the sensor assembly of the imaging system are constructed using appropriate visible and NIR transmitting optical materials and antireflection coatings and are optically designed to transmit visible and NIR images for which chromatic and geometric aberrations are minimized. FIG. 1 depicts a typical configuration of an optical instrument, optical coupler and imaging system such as that being described above.
Although the preponderance of optical instruments currently in use are not optimized for both visible (VIS) and NIR light imaging, such instruments may still transmit sufficient NIR light that it may also be desirable to enable the previously described VIS-NIR imaging system for use with these conventional optical instruments. Conventional optical instruments are typically well-corrected for imaging throughout the visible spectrum, but without equivalent correction in the NIR, NIR images acquired with the aforementioned VIS-NIR imaging system through such optical instruments are likely to be of poor quality. Furthermore, although some of the NIR image aberrations introduced by conventional optical instruments may be corrected by applying compensating lens design techniques to the optical couplers, such techniques are typically not powerful enough to correct both the aberrations and the shift in focal plane between the visible and NIR images produced with such instruments. A novel optical coupler capable of correcting for the optical aberrations and for the difference in visible and NIR focal plane locations introduced when using conventional optical instruments is, consequently, highly desirable.